The most abundant coal resources in western North America are low rank coals, including subbituminous and lignite. Many deposits of the low rank coals are relatively inexpensively mined compared to higher-rank coals in eastern North America, Australia and Europe, but their economic value is significantly reduced because they contain significant amounts of moisture and oxygen in combined form. Moisture contained within the coal results in both increased transportation costs from the coal deposit to the point of use, and decreased heat available from the coal when burned because of the heat required to evaporate the moisture. The problem generally exists in all subbituminous coals and is particularly acute with low-rank coals, which may contain from 20% to 50% moisture when mined.
A well known practice to reduce the moisture content in coal is to evaporate the moisture by low temperature heating of the coal to about 80.degree.-150.degree. C. The low temperature heating method, however, is disadvantageous because the resultant dried coal has a propensity for self heating and also readily reabsorbs moisture from the atmosphere to approach its previous moisture content state. Self-heating, also referred to as "autogenous" heating or pyrophoricity, is the tendency of a material to spontaneously ignite and burn upon exposure to air at ambient conditions. This self heating is related to two processes, the heat of rehydration of the dried coal or char and the chemisorption of oxygen.
Mild gasification methods, used in producing process derived fuel, also typically dry the coal before gasification to form char. The coal is dried by thermal processing using continuously flowing heated streams of oxygen-deficient gas for convective heat transfer to the coal. Similar to dried coal, it is well known that char has a propensity to self-heat when stored and shipped at atmospheric ambient conditions or when exposed to water in liquid or vapor form.
When exposed to the atmosphere, dry char rapidly adsorbs water vapor and oxygen and subsequently heats up and it ignites if not cooled. The adsorption of water vapor or oxygen and resultant oxidation of the char is manifested in an exothermic reaction. Oxygen physically adsorbs onto the surface of the coal and chemically reacts with organic molecules within the coal. This reaction can have an ultimate heat release of about 120,000 kJ per mole of oxygen. Because oxidation rates will approximately double with each 10.degree. C. rise in temperature, the heat, if not dissipated, will promote a self-accelerating oxidation process and cause the coal temperature to rise progressively until the coal spontaneously ignites. If self-heating of the char reaches ignition temperature it is commonly referred to as "spontaneous combustion," which represents a serious hazard whenever substantial amounts of char are stockpiled or transported.
Another cause of self-heating occurs when char adsorbs water, either in liquid or vapor form. At ambient temperatures, carbon oxidation rates are generally too small to initiate the combustion of char. However, when dry coal or char is wetted by water, heat is released due to the adsorption of water onto the dry coal or char. Water vapor physically adsorbs onto the coal or char releasing the heat of vaporization which amounts to about 20,000 kJ/mole of water. Such "heat of wetting" raises the temperature of the dry coal or char to levels at which carbon oxidation occurs more rapidly. The increased oxidation rates eventually lead to spontaneous combustion. This mechanism explains why spontaneous combustion of coal commonly occurs after rain following a period of dry hot weather. The foregoing mechanism also takes effect when dry coal or char is placed on wet ground, and when wet coal is loaded onto an established, partially dried-out stockpile. In the latter cases, heating invariably begins at the interface between the wet and dry material.
Equilibrium moisture is defined by ASTM as the moisture content of a sample of coal or char when it is equilibrated with 96% relative humidity air at 30.degree. C. It is believed that this condition is similar to that found in a stockpile of moist coal. If a stockpile of coal is above its equilibrium moisture level then it will tend to lose moisture to its surroundings, on the other hand, if it is below its equilibrium moisture level then it will tend to pick-up moisture from its surroundings.
Equilibrium moisture plays an important role in the self-heating of coal or char stock piles. If the coal or char are below their equilibrium moisture then a stockpile will tend to pick-up moisture, causing the stockpile to heat up due to the heat of rehydration. The rise in temperature will cause the rate of oxygen chemisorption to increase which will in turn cause the effected part of the stockpile to heat and to eventually self-ignite. Simply, drying low rank coals does not change the equilibrium moisture level, therefore the dried coal will tend to rehydrate back to its equilibrium moisture level releasing the heat of rehydration.
In view of the propensity of char to self-heat, it is desirable that all of the char in a stockpile is suitably treated to passivate the self heating character of the char thereby protecting the remainder of the pile from spontaneous combustion.
It is an object of the present invention to provide a process for treating noncaking coal to form char. Another object of the present invention is to provide a char having a significantly higher heating value than the coal, e.g., 8,500 Btu/lb as compared to 11,500 Btu/lb. It is a further object of the present invention to provide a char having suitable storage stability while retaining desirable fuel characteristics. As used herein the term "low end volatile components" refers to those compounds which are vaporized from about 400.degree.-480.degree. C. Similarly, the term "high end volatile components" refers to those compounds which are vaporized from about 480.degree.-950.degree. C.